Our invention relates to a method of measuring parameters of a crystal filter, and particularly to a method of measuring parameters of a dual resonator crystal filter to determine the resonator frequencies, the synchronous peak separation frequency, and the normalized center frequency of the filter.
Crystal filters have reached a stage of development where they are used extensively in radio and electronic circuits. Dual or two-resonator crystal filters are preferred in many applications because they provide the characteristics of a very narrow bandpass filter or part of a very narrow filter due to the extremely high Q of the associated crystal resonators. Such very narrow filter characteristics are virtually impossible or extremely difficult to realize with lumped devices, such as inductors and capacitors. The coupled dual crystal filter is comprised of only one plate of piezoelectric material.
Accordingly, a primary object of our invention is to provide a new and improved method of measuring selected parameters of a dual resonator crystal filter.
A typical dual resonator crystal filter includes a plate of piezoelectric material having an input electrode and an output electrode on one face of the plate, and a common electrode on the opposite face of the plate. A coupling strip may be positioned on the one face between the input and output electrodes. The input electrode and common electrode form the first resonator of the filter, and the output electrode and common electrode form the second resonator of the filter. The filtering characteristics of the filter are determined primarily by the resonator frequencies of the first and second resonators, by the synchronous peak separation frequency, and by the normalized center frequency. Synchronous peak separation frequency is defined as the absolute value of the difference between the two short circuit resonant frequencies that would occur if the two resonators were tuned to the same resonant frequency. Where, as is typical, these dual resonator crystal filters are mass produced, a reliable, accurate, and rapid method of determining the filter characteristics of a completed filter is needed.
Accordingly, another object of our invention is to provide a new and improved method of measuring parameters of a dual resonator crystal filter to determine the filtering characteristics of that filter.
Another object of our invention is to provide a new and improved method of measuring the parameters of a dual resonator crystal filter so as to permit the filtering characteristics of the filter to be reliably, accurately, and rapidly determined.
Another and relatively specific object of our invention is to provide a new and improved method for measuring parameters of a dual resonator crystal filter so as to permit the filtering characteristics of such a filter to be reliably, accurately, and rapidly determined by formulas, and without the necessity for adjusting or compensating the measuring device used in our method.